When making color emission devices such as televisions or color display elements used in displays for computers, portable telephones and so on by using luminescence elements such as organic EL elements, pixels emitting light of three primary colors, blue, green and red must be formed. There are the following roughly classified three ways for the formation of three pixels but these ways have the following problems.
A. Three Color Applying Way
This way is to arrange blue, green and red luminescence elements on a screen respectively.
However it is difficult to obtain a color emission device with a high efficiency and small change in white balance with time, since the pixels of three colors have different durability, particularly red pixels have a short durability and low efficiency.
B. Combination of White Organic EL Element and Color Filter
This way is to obtain blue, green and red color from white emission containing all the three primary colors by using color filters.
Here, “a color filter” is defined as a filter that cuts off a part of emission components but transmits only a desired component emitted from an organic EL element. In order to obtain a color emission device with a high efficiency and excellent color balance in this way, it is essential to use a white organic EL element having a good balance of emission components of the three primary colors. However in order to contain a red component in higher intensity than blue and green components, an element of very complicated structure is required. As a result, it is difficult to manufacture devices with a good reproducibility.
C. Combination of Blue or Bluish Green Organic EL Element and Color Converting Member (Color Converting Way)
This way is to emit light of the three primary colors by converting blue emission to green and red emissions by color converting members.
In order to obtain a color emission device with a high efficiency and excellent color balance in this way, it is essential to develop color converting members capable of efficiently converting light emitted from an organic EL element, particularly a red converting member, and an organic EL element capable of efficiently emitting blue or bluish green light.
FIG. 16 is a diagram showing a color emission device using the conventional color converting way. In this figure, numeral 10 denotes a color emission device, 12 a blue pixel, 14 a green pixel, 16 a red pixel, 20 an organic EL element emitting blue light, 30 a color converting member containing fluorescent dyes. A fluorescent dye is excited once it is irradiated by light having a certain wavelength, and it converts the light to fluorescence having a longer wavelength. The color converting member 30 contains a red converting member 36 and a green converting member 34. The green converting member 34 converts blue light emitted from the organic EL element 20 to green light. The red converting member 36 converts blue light to red light. That is, part of blue light emitted from the organic EL element 20 is converted to green light and red light by the color converting member 30 to provide pixels of the three primary colors, thereby enabling the formation of color emission device.
However in a color emission device thus constructed, the efficiency of converting blue light to red light is lower than that of converting blue light to green light, since the energy gap is larger when converting blue light to red light. That is, the intensity of red light is lower than those of blue light and green light.
Accordingly, some methods are suggested to enhance a red converting efficiency, thereby increasing the intensity of red light.
For example, JP-A No. H10-039791 discloses that red, blue and green pixels have different area ratios or red pixels have larger area ratio so that the intensity or luminance of red light is enhanced, thereby maintaining white balance for long time. However the arrangement pattern of pixels is complicated since the area ratios of color pixels are different from each other.
It is attempted to make a voltage or current for driving organic EL elements in red pixels higher than those of other pixels, thereby letting the luminance thereof higher than those of organic EL elements in green and blue pixels.
However if only the luminance of red pixels is increased, only the organic EL elements in the red pixels are degraded so that the luminance of red pixels cannot be maintained high.
As stated above, various improvements have been done to increase the efficiency of a blue organic EL element insofar but they reach close to the theoretical limit.
In a red converting member, a fluorescent dye that absorbs an emission component from an organic EL element to emit red fluorescence is dispersed in a transparent binder. However there is no combination of fluorescent dye and binder that can satisfy both high converting efficiency and reliability against environment such as heat and light among known dyes and binders at present. If an attempt is made to obtain a high converting efficiency, the material is inevitably unstable for environment. If an attempt is made to obtain a high reliability, its converting efficiency is inevitably reduced.
Further the degradation of a red converting member adversely affects the color stability of whole the color emission device.
Color filters are effectively used to obtain a sufficient reproducibility since they can remove unnecessary emission components that deteriorate color purity. However in the above method, the luminescent efficiency of an organic EL element and converting efficiency of color converting member reach the limits. Thus, if color filters are used to remove unnecessary light for the improvement of color purity, the luminescent efficiency of a color emission device becomes extremely low.
The invention has been made to solve the above-mentioned problems. An object of the invention is to provide a color emission device where the intensity of red light can be increased, while suppressing polarization of degradation in an organic EL element without the different area ratios of pixels from each other.
Another object of the invention is to provide a color emission device with excellent color purity, luminescent efficiency and long durability.
The inventors have been made efforts. As a result they made the invention by the finding that the intensity of red light can be increased by letting the emission spectrum peak of an organic EL element to coincide with the excitation spectrum peak of a red converting member.
Further they made the invention by the finding that the color purity, luminescent efficiency and durability of a color emission device can be enhanced by combining color converting members with a luminescence element emitting light that has a blue component and a yellow to red component at a luminescence intensity ratio of 9:1 to 5:5.